Milky Way

Aperture

Aperture is the diameter of a telescope's primary mirror or lens listed in millimeters or inches.  The bigger the aperture of a telescope, the more light it will gather, allowing the observer to see more detail on celestial objects and ascertain finer details that a telescope of lesser aperture may not see.

Astrophotography

This refers to photography of astronomical bodies and phenomena. Astrophotography is not new, for example the popular T threading still used today harkens from Tamron’s T-mount developed for their 35 mm cameras - however it has seen a notable increase in popularity with improvements in cameras, mounts, filters, and software making astrophotography much more accessible. This is not limited to celestial bodies such as nebulae, planets, or galaxies either, as solar imaging is now more within the reach of the average consumer than ever before.

Bortle Scale

The Bortle scale measures how light polluted a particular area is and classifies the level of brightness from 1 to 9, with 9 being the brightest. This scale is incredibly helpful for astronomers, as the darker the sky, the more celestial objects are able to be discerned.

Calibration Frames

Calibration frames are types of images that are applied to light frames to remove unwanted artifacts caused by a number of different factors. Dark frames are images taken at the same temperature as the light frames and are necessary to remove hot pixels or other fixed noise for a cleaner image. Bias frames are similar in this aspect, though their purpose is to reduce the sensor’s readout noise seen within the light frames. Another type of calibration frame are flat frames, which remove dust motes or unwanted gradients for a uniform, even field.

Chromatic Aberration

Different wavelengths of light travel at different speeds based on the medium it occupies. When white light is exposed to glass such in a telescope or lens, blue light, red light, and green light slow at varying rates. This change of speed causes each wavelength to come to focus at different points along the focal plane, resulting in color fringing seen within the images taken.

Dark Frame

A dark frame is a type of calibration frame that aims to subtract sensor heat noise and hot pixels from your final image. These frames are taken at the same exposure length and ISO/gain settings as your light frames, though are taken with the lens cap or telescope cover on instead. In general, the more dark frames you take to subtract from the final image, the better.

Dedicated Astronomy Camera

These cameras don’t look like what one traditionally thinks of when imaging a camera; instead taking the form of cylinders or pucks, with no physical controls, displays, or viewfinders to speak of. These require a computer or WiFi control device to take images, with more advanced models additionally requiring external power. What they give in return for all of these concessions is granular control over the sensor settings, increased sensitivity to wavelengths that more traditional cameras filter out, options for deBayered sensors (true monochrome), designs that easily connect with astronomy equipment, and in some cases cooling for increased performance.

Dobsonian Telescope

The Dobsonian telescope consists of a Newtonian reflector optical tube assembly mounted on a very simple alt-azimuth box-style mount with a lazy susan base. This base was invented by John Dobson to encourage people to make their own telescopes from start to finish. Up until that time, telescopes required a heavy equatorial mount that produced a financial or weight barrier for some otherwise enthusiastic budding astronomers. John Dobson and his base changed all that, and even today, Dobsonian telescopes still provide the most light-gathering dollar for dollar.

Earth

Also known as the Pale Blue Dot, Earth is the unique planet we are standing on right now. It's located between Venus and Mars in the goldilocks zone of our solar system, making it the third planet from the sun. Our Earth is estimated to be around four billion years old and is currently the only known planet to support the necessary conditions for life.

EdgeHD Telescope

EdgeHD is Celestron's top of the line flat field aplanatic Schmidt-Cassegrain telescope series. You can buy a variety of apertures, from 8 to 14 inches, as an OTA or as part of a telescope/mount package. EdgeHD telescopes are great for visual use, but they really shine for astrophotography. With a wide field of view that is three times flatter than a standard SCT, you will see beautiful, pinpoint stars across the entire field, even if you have a full frame (35 mm) CMOS or CCD camera sensor to satisfy.

Equatorial Mount

An equatorial mount is an astronomy instrument that features two axes of rotation: right ascension (RA) and declination (DEC). Equatorial mounts also feature an additional axis, called the polar axis, that these RA and DEC axes rotate about. This polar axis is to be lined up with Earth’s celestial pole to accurately counteract Earth’s rotation. These mounts are ideal for astrophotography applications, as the addition of a polar axis eliminates the issue of field rotation within captured images.

Exposure Time

Exposure time is the amount of time the camera sensor is allowed to collect light. In general, the longer the exposure time, the more light collected, and the brighter the image will become. This should be selected with caution though, as an exposure time that's too long can oversaturate the pixels and blow out the image, resulting in a loss of signal. Determining the correct exposure time is highly dependent on the aperture of the optics as well as the gain settings used. A larger aperture will produce a brighter image than that of a smaller aperture with the same exposure time. In a similar fashion, an image with a higher gain setting will be brighter than a lower gain setting image with equal exposure time. Finding the perfect balance between the aperture, gain, and exposure time will maximize image quality.

Extra-Low Dispersion (ED) Glass

ED stands for “extra-low dispersion” and is a type of glass designed to specifically reduce the separation of wavelengths when light passes through. Coatings of magnesium fluoride, zirconium dioxide, calcium fluoride, and titanium dioxide help bridge the gap between the different wavelengths of light, providing a singular focal point for all wavelengths. This helps reduce chromatic aberration and in turn provides heightened optical performance. Also, the more coatings on the lenses, the higher the transmission rate becomes for brighter, more detailed views and images.

Eyepiece

An eyepiece is a group of lenses housed in a small package that is closest to the eye when used with a telescope, microscope, or spotting scope. The eyepiece provides a particular magnification when paired with a telescope, therefore most amateur astronomers use a variety of eyepieces to change magnification for different types of objects. The eyepiece nomenclature is expressed in its focal length in millimeters. To figure out the magnification, simply divide the focal length of the eyepiece into the focal length of the telescope. The result is the magnification provided in your particular telescope or one with the same focal length.

Field of View (FOV)

In simple terms, your field of view is the amount of sky that is witnessed by your telescope/camera combination, or telescope/eyepiece combination. This measurement is calculated in angular degrees. To calculate how much of the sky you can image with your astrophotography rig, take the width of your camera chip, multiply it by 57.3, then divide that product by the focal length of your optics. If you want to determine how much of the sky you can view through your eyepiece, take the apparent field of view of your eyepiece (provided by the manufacturer), then divide it by the quotient of your telescope’s focal length & the focal length of your eyepiece.

Finder Scope

A finder scope fits on top of the main telescope and is used to help you find and center objects in your eyepiece. A finder can be as simple as a red dot finder or it can be a high quality small telescope in its own right.

Focal Length

The focal length is the distance, usually measured in millimeters, between the primary mirror or lens and the point at which the image comes to focus.  Generally, classic refractors have a longer focal length, Newtonian reflectors tend to have a focal length that is shorter, and Schmidt-Cassegrain fall somewhere in the middle.

Focal Ratio

The focal ratio is calculated by dividing the aperture (mm) of the primary mirror or lens into the focal length. Example: 2500 mm divided by 254 mm (10") equals an f/ratio of 9.84, which is usually rounded off, in this case to f/10. The focal ratio signifies how quickly a telescope gathers light and tells us something about the telescope's field of view, how long exposures will take during astrophotography sessions, and how much magnification the eyepiece will produce for that telescope.

Hemisphere

A hemisphere is a mathematical term that describes half of a sphere or globe. While the dividing line between hemispheres can be arbitrary, when talking about our planet or the celestial sphere, we tend to divide the hemispheres into west, east, north, and south.

Highest Useful Magnification

The term Highest Useful Magnification is used by telescope manufacturers to describe the most magnification you can typically be expected to use on a normal night and still bring an image to sharp focus. A basic rule of thumb for maximum magnification is 40X-50X per inch of aperture, with max magnifications generally topping out at 500X or so regardless of the aperture. You will find that maximums vary depending on the night. Observing conditions change constantly and will cause a once sharp view to become blurry or allow a blurry view to clear up in seconds. It is the nature of telescope observing on a planet with an atmosphere.

Image Circle

This specification refers to the area where an image is formed by the optics/ corrective element. This is useful for assessing whether a certain camera sensor size will be supported by the optics, or if vignetting/ poor performance will be seen in frame.

Imaging Newtonian

An Imaging Newtonian is a fast reflector telescope that is normally optimized for astrophotography or astro-imaging. Most Imaging Newtonian telescopes have an f/5 focal ratio or less, and some are not meant for visual use at all but rather, were designed to be dedicated imaging telescopes. It is common to find fast imaging Newtonian reflectors for sale as optical tube assemblies only, allowing the astro-imager to use his choice of equatorial mount.

Interstellar Clouds

Interstellar clouds are a collection of various elemental gasses, dust, and plasma left over from various galactic events, such has supernovas, galactic collisions, and even the big bang.

Light Pollution

Light pollution is the brightening of the atmosphere due to lights from streetlamps, other forms of artificial light, and even the Moon. As light enters the atmosphere, it washes out the night sky, making it very difficult to observe the stars, nebulae, and planets. In order to combat light pollution in astrophotography, special filters have been developed to cut through excess light and enhance images. These filters are known as City Light Suppression filters, commonly referred to as CLS filters.

Luminosity

Luminosity is when an object produces or reflects light and appears to shine. For astronomy, this refers to the radiant power emitted, reflected, or received per unit of time.

Megapixels

A megapixel is one million pixels and refers to the number of pixels a sensor contains. For example, if a sensor has two million pixels, then this would be a 2-megapixel (2MP) sensor. These pixels capture light and color from the subject, and when combined, the signal collected results in an image. This measurement of the pixels available determines the resolution of a sensor and overall quality of the image produced. In general, the more pixels a sensor has, the higher resolution images that sensor will yield.

Milky Way

The Milky Way is the name of the galaxy that hosts our solar system. Named for its bright milky appearance in the night sky, the Milky Way is home to countless stars, nebulae, and even a few supermassive black holes. It estimated to be over 13 billion years old, and it would take over 100,000 light years to travel from one side to the other.

Modified DSLR / Mirrorless Camera

A type of DSLR or mirrorless camera that has had a filtering component removed. This component, a piece of glass, is put in place to block out frequencies that would otherwise make “regular” images look unnatural as modern sensors are sensitive to a wider range of frequencies than the human eye can perceive. However, there are some frequencies on the edge of this range that are important to astrophotography (mainly Ha), which these filters block to some degree. Accordingly, removal of this filter will provide better astrophotography performance, but impact “regular” photo taking unless a clip-in or lens filter that filters these frequencies is used. Given the difficulty of removing this filter, this is something most users need to work with 3rd parties on.

OTA

The acronym OTA stands for Optical Tube Assembly. An OTA is simply the telescope portion of a telescope/mount/tripod package. Some telescope users prefer to buy the OTA separately so they can create a custom astrophotography set-up or use a mount they already own.

Optics

In basic terms, this refers to the system that “collects” and focuses incoming light down to a smaller, brighter, magnified image that is then viewed through an eyepiece or captured by a camera. There are different ways to accomplish this, most commonly with glass elements (refractor) or with systems of mirrors (reflector). Each approach comes with its own set of tradeoffs, whether that be chromatic aberration, coma, etc. which can sometimes be mitigated with upgraded optical components (ED glass) or additional corrective components (like coma correctors). Numbers of standard optical designs have emerged as well, such as doublets, triplets, Petzvals, Newtonians, SCTs, RCs, and more.

Orbit

An orbit is a predictable and periodic path an object follows as it moves through the space-time warped by a more massive celestial object. We have known about celestial orbits since early antiquity, however, it was not until Johannes Kepler formalized the three laws of planetary motion that we gained the ability to acutely predict the movements of our planetary neighbors

Pixel

Digital images are made up of many, many, small boxes arranged in a grid, and these boxes are called pixels. To create these digital images, camera sensors are similarly made up of thousands or millions of small light-measuring components arranged in a grid. These too are referred to as pixels.

Polar Alignment

Polar alignment is the process of aligning a telescope mount’s polar axis with the Earth’s axis of rotation. By having these two axes parallel to one another, precise counteraction of the Earth’s rotation can then be achieved. While a typical process of equatorial mounts that have three inherent axes of rotation, a similar effect can also be achieved by utilizing an equatorial wedge with two-axis alt-azimuth mounts.

Polar Scope

Polar scopes are small telescopes that assist with aligning a mount’s polar axis with the Earth’s axis of rotation. They are found within your mount and are fitted with an internal reticle that shows Polaris’s position in reference to the true celestial North Pole, and Sigma Octantis’s position in reference to the true celestial South Pole. Through alignment of these pole stars within the polar scope, the mount will then be accurately polar aligned.

Reflector Telescope

A reflector is a telescope design in which mirrors are used to gather and focus light. Reflector telescopes are commonly called Newtonian Reflectors, or simply a Newtonian in deference to their inventor, Sir Isaac Newton.

Refractor Telescope

A refractor is a telescope design that uses lenses to gather and focus light. While there are some exceptions to the rule, achromatic refractors use two lenses in their design, and while they are economical, achromats are only capable of focusing two out of three wavelengths of light. An APO telescope, or apochromatic refractor, uses 3 lenses to bring all three wavelengths of light to a single focus to produce an image virtually free of extraneous color.

SCT

The acronym SCT stands for Schmidt-Cassegrain Telescope, one of the most popular telescope designs in amateur astronomy today. A Schmidt-Cassegrain, which belongs more broadly to the Catadioptric telescope type, uses a folded optical design incorporating both mirrors and lenses to gather and bring the light to focus. The folded light path allows for a short tube assembly even with relatively large apertures of 8" or more. A shorter tube length makes the SCT far more portable than a classic Newtonian or refractor of the same aperture.

Seeing Conditions

This term refers to the overall clarity of the night sky at any given time and within any given location. This clarity fluctuates constantly, as it is based on numerous different atmospheric conditions, such as humidity, turbulence, high clouds, heat, and plenty more. To give an example, you may have witnessed stars in the night sky “twinkle.” This is due to our atmosphere distorting their incoming light, causing a twinkling effect. When it comes to astronomy, especially when observing the planets and the Moon, seeing conditions are very important to consider, as these distortions can cause the subjects to become blurred and unfocused. It’s best practice to observe celestial objects when the atmosphere is most stable.

Sensor

A camera sensor is the main component of a DSLR, deep sky camera, or smart telescope that turns incoming light into data that can then be displayed on digital devices like mobile phones or computers. Accordingly, these are just as important as the optics of a system for a quality image. There are a wide variety of specifications that are important for a sensor - physical size, pixel size, resolution, dynamic range, amounts of image “noise”, and other technologies such as back-side illumination. As a result, there is also a wide variety of sensors available that attempt to balance these in differing ways for different applications and price points.

Sensor Size (Full Frame, APS-C, Micro 4/3, Etc)

Imaging sensors come in many different proportions, such as 4:3 or 1:1, and physical sizes. There are some common combinations that have received a name - full-frame, APS-C, micro four thirds (4/3). The most important component of sensor size for astrophotography is the measurement from corner to corner (diagonal) of the sensor, which can then be compared to a telescope/ corrective element's image circle to assess how well the two may pair.

Sky Glow

As light interacts with the various component chemicals that make up our atmosphere, it occasionally changes direction and intensity. We perceive this interaction as a reduction in the transparency of our atmosphere, which can impact the visibility of objects in the night sky. Sky glow can be caused by terrestrial light pollution, sunlight, and even moonlight.

Star Tracker

A star tracker is a mini equatorial mount that has a motorized RA and static DEC axis. They are designed with portability in mind, and as such, are very lightweight and compact. They’re perfect for tucking away in a backpack, and many even run on batteries for heightened portability. While optimal for Milky Way photography and small imaging trains, some models are robust enough for carrying small refractors.

Star Trail

Star trails are a phenomenon that occurs when taking images of the sky using long exposures. These images depict the apparent motion of the stars in the sky when imaged without tracking.

Stars

A star is a luminous sphere of plasma held together by the collective gravitational attraction of hydrogen and helium atoms. Stars also contain various trace elements, such as lithium, carbon, and towards the end of their life cycle, iron. Stars come in many temperatures, colors, and sizes. High mass stars like Betelgeuse appear to shine red from our perspective because our eyes are only sensitive to a small portion of the electromagnetic spectrum. To see the true luminosity of Betelgeuse, you need specialized equipment designed to capture the appropriate wavelength of light.

Tracking

As the Earth is continuously spinning and in motion, the location of a celestial object in the sky moves over the course of a night. This becomes apparent during observation as a target moves out of view, and particularly observable in images as stars and objects quickly become a blur as exposure time and focal length increases. To compensate for this, computerized mounts and smart telescopes employ tracking techniques to keep the target centered in the optics. Depending on the motion style of the mount, the resulting image can vary. Alt-Az motion, popular in smart telescopes, keeps the object centered but cannot compensate for its “spin” without what is known as a wedge. Accordingly, these images lose information on the edges of the frame as they rotate out of view, leading to ever smaller, circular, images as time spent imaging increases.

Tracking Modes

As opposed to the tracking rate that determines the speed at which the mount will track the sky, the tracking mode of a telescope mount determines the way in which the mount will counteract the Earth’s rotation. This can be selected from either the hand controller or software. Typically, you will find the modes of: EQ North, EQ South, Off, and Alt-Azimuth. For instance, if you are polar aligned with the north celestial pole, choosing EQ North will be the appropriate tracking mode to select, while EQ South is reserved for those polar aligned with the south celestial pole. Equatorial tracking modes are optimal for astrophotography applications, while the alt-azimuth mode is ideal for those who are conducting visual astronomy.

Tracking Rates

Unlike the tracking mode which denotes the way in which the mount will track the sky, tracking rates determine the speed at which the mount will track the sky. This can be selected from the mount’s hand controller or through computer software and should be selected based on the target that is being tracked. Typically, there are three tracking rates: Sidereal, Lunar, and Solar. The sidereal tracking rate moves the mount at a speed that is equivalent in magnitude, yet opposite in direction as the speed of Earth’s rotation. This is ideal for all targets within the night sky save the Moon and the Sun, in which the lunar and solar tracking rates are appropriate for.

Tripod

A tripod is a tool that consists of three legs of varying thickness and materials, and is used to support cameras, telescope mounts, and other equipment through (most often) a threaded connection. It acts as a stand to increase height and stability.

Vignetting

Vignetting is seen as the darkening of the corners of the frame within an image. This happens when the camera sensor is not exposed to enough light, resulting in a shadow effect along the borders of the image. This issue presents itself for a number of reasons, though most commonly occurs when using incompatible sensor sizes and image circles and using too small filters for the imaging assembly.

Waning Moon

As the Moon begins to move towards the sun in its orbit around the earth, the illuminated side appears to shrink with each passing day. This period of the lunar cycle is labeled “waning” to distinguish it from the opposing, yet similar, waxing phases.

Waxing Moon

As the Moon begins to move away from the sun in its orbit around the earth, the illuminated side appears to grow larger each passing day. This period of the lunar cycle is labeled “waxing” to distinguish it from the opposing, yet similar, waning phases.